Additional to the breakdown provided above, those sentances have two slightly different meanings:

"Which chair is it" is an open question requesting additional information.
E.g. There are several chairs and you have no current idea of which is specified.

"Is it this chair" is a closed question that has already formed a presumption and is requesting affirmation. E.g. You've already "chosen" a chair that might be the correct one whereas in the prior question you havent.

The English here is confusing. Is it the archaic/formal way of asking "who does this chair belong to?", or is it more straightforward "which of these chairs did you mean?"?
I'm leaning toward the later, but still unsure.